Allow vold, healthd, slideshow, and watchdogd access to /dev/kmsg.
These processes log to the kernel dmesg ring buffer, so they need
write access to that file.

Addresses the following denials:

    avc: denied { write } for pid=134 comm="watchdogd" name="kmsg" dev="tmpfs" ino=9248 scontext=u:r:watchdogd:s0 tcontext=u:object_r:kmsg_device:s0 tclass=chr_file permissive=0
    avc: denied { write } for pid=166 comm="healthd" name="kmsg" dev="tmpfs" ino=9248 scontext=u:r:healthd:s0 tcontext=u:object_r:kmsg_device:s0 tclass=chr_file permissive=0
    avc: denied { write } for pid=180 comm="vold" name="kmsg" dev="tmpfs" ino=9248 scontext=u:r:vold:s0 tcontext=u:object_r:kmsg_device:s0 tclass=chr_file permissive=0

These denials were triggered by the change in
https://android-review.googlesource.com/151209 . Prior to that change,
any code which called klog_init would (unnecessarily) create the
device node themselves, rather than using the already existing device
node.

Drop special /dev/__null__ handling from watchdogd. As of
https://android-review.googlesource.com/148288 , watchdogd no longer
creates it's own /dev/null device, so it's unnecessary for us
to allow for it.

Drop mknod from healthd, slideshow, and watchdogd. healthd and slideshow
only needed mknod to create /dev/__kmsg__, which is now obsolete.
watchdogd only needed mknod to create /dev/__kmsg__ and /dev/__null__,
which again is now obsolete.

(cherry picked from e2651972)

Bug: 21242418
Change-Id: If01c8001084575e7441253f0fa8b4179ae33f534

Add wakelock_use to slideshow.te to fix the following denial:

avc:  denied  { block_suspend } for  pid=137 comm="slideshow" capability=36  scontext=u:r:slideshow:s0 tcontext=u:r:slideshow:s0 tclass=capability2 permissive=0

Change-Id: If84f167cd235e8196eadf3fb85cc725a5ea464e6

Add rules to allow /sbin/slideshow to access framebuffer and input
devices at early stages of boot, and rules to allow init to execute
the program (from init.rc using exec).

Needed by changes from
  I58c79a7f3ac747eec0d73a10f018d3d8ade9df7d

Change-Id: I1d5018feb7025853f0bf81651f497fef8c3a6ab0